1. Field of the Invention
The invention concerns the preparation of dispersions of polyesters of high molecular weight which have a crystalline or partially crystalline character due to the arrangement of the polyester chains.
2. Discussion of Prior Art
Nonaqueous dispersions of polymers have long been known. They are described, for example, in a review by R. Dowbenko and D. P. Hart, "Nonaqueous Dispersions as Vehicles for Polymer Coatings," Ind. Eng. Prod. Res. Develop., Vol. 12, No. 1, 1973, pp. 14-28. These dispersions are always prepared by the polymerization of monomers containing olefinic double bonds, with the aid of liquids which dissolve the monomers but not the polymers. Such a method, however, is not applicable when the monomers, which are already dispersed, are to be reacted to a high molecular weight, not by polymerization but by thermal polycondensation.
For the purpose of converting highly polymerized substances into dispersions, it has already been proposed to dissolve especially polyethylene or ethylene copolymers in a suitable liquid with the application of heat, and to reprecipitate them by cooling, thereby obtaining relatively fine powder after filtration (French Pat. No. 945,962). If the cooling takes place very rapidly and the concentration of the polymer amounts to no more than 8% and 5%, respectively, very finely divided, stable dispersions are obtained (German Offenlegungsschrift No. 2,814,030).
This last-named method has the disadvantages that expensive apparatus are required for the purpose of the very rapid cooling, and that dispersions can be obtained having only an extremely low solids content, the use of which is uneconomical.
When the polymers that are to be dispersed are brittle hard resins, it is also possible to make them dispersible by mechanical comminution (cf., for example, German Offenlegungsschrift No. 2,210,484). In the case of high polymers, the achievement of the required degree of fineness by grinding is generally impossible, even when the material is chilled with liquid nitrogen, or else it can be accomplished, at hardly acceptable expense, by sifting the resin and constantly recycling the particles that are still not sufficient.
It is furthermore known that amorphous polyesters of high molecular weight in combination with suitable triazine resins, dissolved in suitable varnish solvents, will, upon being baked onto sheet metals, yield sterilizable coatings (German Offenlegungsschrifts Nos. 1,807,776 and 2,126,048). This sterilizability, however, is generally satisfactory only if pure water is used as the sterilizing liquid. If the water used as the sterilizing liquid, however, contains additive acids used in foods, such as lactic acid, citric acid, tartaric acid or acetic acid, the coatings may no longer be sufficiently stable during sterilization.
In contrast to the amorphous polyesters used exclusively in accordance with German Offenlegungsschrift Nos. 1,807,776 and 2,126,048 for reasons of solubility, crystalline polyesters, such as those described for example in German Auslegeschrift No. 2,346,559, yield coatings of far superior sterilizability.
These crystalline polyesters, however, due to their basically irremediable insolubility in conventional varnish solvents such as esters, alcohols, ketones and hydrocarbons, have not been able to be used heretofore as varnish raw materials, and they are used for surface coating only by the known powder melting methods, especially fluidized bed sintering methods or the electrostatic powder spraying method. These methods, however, are not applicable in all cases, and this is particularly disadvantageous if it is desired to take advantage of their superior sterilizability, as for example in the lining of food cans or in the coating of sheet metal from which food cans are to be manufactured.